1. Field of the Invention
The present invention relates to a scanning optical device and an image forming apparatus using the same. In particular, the present invention relates to a scanning optical device suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multifunction printer, which has, for example, an electrophotographic process, in which a light flux emitted from light source means is deflected on an optical deflector (polygon mirror) serving as a deflection element and then a surface to be scanned is scanned with the light flux through a scanning optical element having an fθ characteristic to thereby record image information.
2. Related Background Art
Up to now, in a scanning optical device used for a laser beam printer (LBP) or the like, image recording is conducted through the following process. A light flux which is modulated according to an image signal in light source means and emitted therefrom is periodically deflected on, for example, an optical deflector composed of a rotating polygonal mirror (polygon mirror). Then, the deflected light flux is converged in a spot shape onto a photosensitive recording medium (photosensitive drum) by a scanning optical element having an fθ characteristic, to thereby scan the surface of the recording medium is scanned with the light flux.
FIG. 8 is a schematic view showing a main part of a conventional scanning optical device. In FIG. 8, a divergent light flux emitted from light source means 91 is converted into a substantially parallel light flux by a collimator lens 92. The substantially parallel light flux (the amount of light) is limited by an aperture stop 93 and incident into a cylindrical lens 94 having refractive power only in the sub scanning direction. Of the substantially parallel light flux incident into the cylindrical lens 94, a light flux within the main scanning section exits from the cylindrical lens 94 without being changed in its optical state; a light flux within the sub scanning section is converged and imaged as a substantially linear image near a deflection surface 95a of an optical deflector 95 composed of a rotating polygonal mirror (polygon mirror).
The light flux which is reflected and deflected on the reflection surface 95a of the optical deflector 95 is guided onto a photosensitive drum surface 97 serving as a surface to be scanned through a scanning optical element (scanning lens system) 96 having an fθ characteristic while the optical deflector 95 is rotated in a direction indicated by an arrow A. Accordingly, the photosensitive drum surface 97 is scanned with the light flux in a direction indicated by an arrow B (main scanning direction) to thereby record image information.
At this time, a part of the light flux (BD light flux), which is reflected and deflected on the optical deflector 95, is returned using a synchronous detection mirror (BD mirror) 81 through the scanning lens system 96 and incident into synchronous detection unit 84. The synchronous detection unit 84 includes: a slit (BD slit) 82 serving as synchronous position determining means which is located at a position optically equivalent to the photosensitive drum surface 97; and a synchronous detection element (BD sensor) 83. A timing of a writing start position (scanning start position) on the photosensitive drum surface 97 in the main scanning direction is adjusted based on a synchronous signal obtained from the synchronous detection unit 84. Therefore, a scanning line is produced on the photosensitive drum surface 97 to conduct image recording.
In recent years, with a tendency of size reduction of the main body of the image forming apparatus, a compact optical system is required for the scanning optical device. In particular, the synchronous detection optical system (BD optical system) that adjusts the timing of the scanning start position is desired to be more compact because of a limitation on leading of electric wirings from the BD sensor and a limitation on arrangements in a casing (optical box).
In general, the BD optical system uses a portion (end) of the scanning lens, images a part of the light flux (BD light flux) deflected on the optical deflector at a position where the BD sensor is disposed or in the vicinity thereof to adjust the timing of the scanning start position.
An example in which a size of such BD optical system is reduced is disclosed in, for example, Japanese Patent No. 3254367. Japanese Patent No. 3254367 discloses an optical system in which the synchronous detection optical element (BD lens) is composed of an independent anamorphic lens and the BD lens is disposed at a position further apart from the optical deflector than the scanning lens.
The optical system according to Japanese Patent No. 3254367 is effective in reducing a size of the BD optical system and a cost thereof. However, because the BD lens is disposed at a position farther apart from the optical deflector than the scanning lens, the distance between the BD lens and the BD sensor becomes shorter. Thus, there is a problem in that the synchronous detection cannot be conducted with high precision.